The attack took place late Saturday during a celebration at the temple (Gurudwara), The Independent.UK reported.
Spokesman of Essen police Lars Lindemann said the explosion was "quite violent", blowing out several windows. One of the injured was said to be in a serious condition.
The media reported that three people were arrested in connection with the incident.
Sikh groups said the incident took place as people were celebrating the festival of Baisakhi.
Lindemann said the police were working on the assumption that the explosion was caused deliberately but there were no indications of a terrorist incide.� (IANS)